This invention relates to an electric power steering apparatus for applying an assist steering torque to the steering system of a vehicle by an electric motor, and more particularly to an electric power steering apparatus to improve back-up measure at the time of ON failure in a relay circuit provided between a driving circuit of the electric motor and a power supply.
In recent years, a so-called electric power steering apparatus has become more popular as the power steering apparatus of a vehicle. An electric power steering apparatus reduces the steering force required by a driver of the vehicle by applying an assist torque to the steering system with an electric motor during operation of the vehicle.
An electric power steering apparatus, basically, comprises a steering torque sensor for detecting the steering torque of the steering system generated by a steering wheel, a controller for outputting a control signal according to the direction and magnitude of a detection torque, and an electric motor driving circuit for driving the electric motor according to the control signal from the controller.
In the electric power steering apparatus, a relay circuit is provided between the electric motor driving circuit and a power supply. This relay circuit is configured such that ON/OFF of the relay circuit is available by the instruction of the controller and the relay circuit becomes OFF when the controller fails (See Japanese Utility Model publication No. H5-42705).
However, sometimes so-called ON failure arises in the relay circuit wherein the steering joints lock or stick together. In ON failure, the electric power steering apparatus performs a function to address the ON failure in a case where the electric motor driving circuit is in a normal mode. However, there may be a problem in a case where failure arises in the electric motor driving circuit when the ON failure occurs, for example when excess current is sent to the electric motor or there is a short circuit in the power supply circuit or the like failure in the electric motor driving circuit.
So, in a conventional electric power steering apparatus, occurrence of failure in the relay circuit is checked as an initial check at the time that the ignition switch turns ON. The function of the electric power steering apparatus is instantaneously stopped as the electric motor driving circuit enters the OFF state.
Once the function of the electric power steering apparatus is instantaneously stopped at the time ON failure of the relay circuit, manual steering torque of the steering wheel becomes extremely heavy due to a lack of the assist steering torque provided by the conventional electric power steering apparatus. In particular, great manual steering torque is required for a vehicle with a large load applied to a front shaft of the vehicle, and thereby manual steering by the steering wheel becomes difficult.
This invention has been made to address the above problems, and it is therefore the object of the invention to provide an electric power steering apparatus which is capable of manual steering by a steering wheel during ON failure in a relay circuit as a back-up measure. Another object of the invention is to provide an electric power steering apparatus which comprises:
(1) an electric motor for applying an assist steering torque to the steering system of a vehicle;
(2) an electric motor driving circuit for driving the electric motor;
(3) a steering torque sensor for detecting the steering torque of the steering system;
(4) a controller for outputting a control signal based on a detection signal of the steering torque sensor of the electric motor driving circuit to control an assist steering torque with the electric motor; and
(5) a relay circuit, which is capable of switching ON/OFF according to instructions of the controller, which is provided between the electric motor driving circuit and a power supply, wherein the controller includes
(a) a relay failure detection unit for detecting ON failure in the relay circuit; and
(b) a relay failure correction unit for decreasing the assist steering torque, at least the maximum value of the assist steering torque, of the electric motor based on the failure detection signal of the relay failure detection unit at the time of ON failure of the relay circuit.
In the electric power steering apparatus of the present invention, the relay circuit turns ON according to the instruction of the controller, and connects the electric motor driving circuit and the electric motor to a power supply. The steering torque sensor detects the steering torque of the steering system with operation of the steering wheel and outputs the control signal based on this steering torque to the electric motor driving circuit to control the assist steering torque applied to the steering system by the electric motor. When the relay failure detection unit detects ON failure of the relay circuit, the relay failure correction unit decreases the maximum value of the assist steering torque by the electric motor based on the failure detection signal. As a result of this, although the steering torque by the steering wheel becomes heavier than the steering torque at the time of normal mode, manual steering becomes possible even for a vehicle with a large load applied to the front shaft.
The steering torque of the steering wheel becomes gradually heavier for every restart of the vehicle after ON failure of the relay circuit in a case where the relay failure correction unit is configured so as to gradually increase the amount of the decreased assist steering torque according to the number of accumulation input times of the ON signal of an ignition switch after ON failure arises in the relay circuit in the electric power steering apparatus.
In the electric power steering apparatus of the present invention, the relay failure correction unit may be configured so as to multiply a signal value, the signal value being set with the controller, by a correction coefficient having a decimal value, which is less than one, to decrease the maximum value of the assist steering torque by the electric motor based on the detection signal of the steering torque sensor. The relay failure correction unit may be configured so as to decrease the maximum value of the assist steering torque within a limit of the maximum value of the signal value.
The above and other objects, features, and advantages of the invention will become more apparent from the following description when taken in conjunction with the accompanying drawings.